The present invention relates to an optical fiber coupler for adding a lightwave signal to or receiving a lightwave signal from an optical fiber.
One impediment to widespread application of optical fibers in control and communication systems is the limitation in the number of taps in the system because of signal strength losses and reliability problems.
To produce an optical fiber network, such as a local area network, it is generally necessary to interrupt a main line optical fiber, introduce a discrete device and reconnect the main line fiber. The new device has an entry port and an exit port where alignment, insertion and other effects almost invariable produce an excessive dissipation of signal strength at each interface. These losses have proved to be prohibitively high for many applications.
The local network topologies known as the "bus" and "tree" systems suffer from dynamic range limitations and questionable reliability, both features stemming from the aforesaid problems associated with discrete taps. The dynamic range of a bus or tree short haul network might typically be 20dB. With discrete connections geneally offering excess signal losses of 0.5 dB or more per connection, and with two connections per tap, the power budget is quickly consumed in just a few taps, withou even considering the actual tapped signal strength.
In the local network topology known as the "star" system, each node access point is addressed by a dedicated fiber which runs back to a centrally disposed star coupler whose function it is to distribute incoming signals equally to all nodes. The star coupler typically has a predesignated number of ports between which any incoming signal will be evenly divided, regardless of whether or not each port is matched to a node. Very often, more ports will be specified than actually required in order to permit future scale-up. This often puts a strain on the dynamic range capabilities of the system which sees its power budget immediately divided by the number of star coupler ports. If the number of star coupler ports are specified to match existing needs, future expansion will result in problems similar to those discussed with reference to the bus and tree systems.
Several patents disclose side-by-side positioning of optical fibers or waveguides to couple light into and out of main line optical fibers or waveguides without the need to sever the main line optical fibers.
U.S. Pats. Nos. 3,936,631 and 3,982,123 disclose the positioning of a disk-like coupling body between an optical fiber and a photodetector for tapping a portion of the light signal from the fiber.
U.S. Pat. No. 4,021,097 discloses the coupling of an optical fiber with a slab of light propagating material. The fiber cladding is removed in the coupling region and the fiber has a negative curvature which leaves the fiber coupling region under tension.
U.S. Pat. No. 4,355,863 discloses the bundling of optical fibers in which a portion of cladding has been removed. A common coupling zone in the fiber bundle permits light coupling between the fibers.
U.S. Pat. No. 4,387,954 discloses an evanescent wave coupler in which two optical fibers which have a portion of the cladding removed are juxtaposed with an interleaf film between them. The interleaf film secures a constant spatial relationship between the fibers to permit evanescent coupling therebetween.
U.S. Pat. No. 4,264,126 discloses an optical fiber coupler in which a pair of optical fibers with their cladding removed are braided in tension and then placed in a coupling solution.
U.S. Pat. No. 4,087,156 discloses an optical fiber transmission mixer wherein the cladding material is removed from an intermediate region of a plurality of fibers and the exposed fiber cores are encapsulated in a matching or slightly higher refractive index material. The encapsulating region is then enclosed in low index sheath to prevent light from escaping from the mixer.
The prior art couplers which utilize cladding removal are generally low in efficiency, mechanically suspect, and often limited in input or output modes.
It is therefore an object of the present invention to provide an optical fiber coupler which has reduced light signal loss.
It is another object of the present invention to provide an optical fiber coupler which has improved mechanical reliability.
It is a further object of the present invention to provide an optical fiber coupler which may be installed in the field and without severing the bus optical fiber.
It is another object of the present invention to provide an optical fiber coupler which may be used simultaneously in both input and output modes.